The present invention relates to non-steady-state photo-induced electromotive force (EMF) detectors and to a class of detector with, potentially, an improved field of view (FOV), bandwidth and photon collection efficiency. This is a class of adaptive detectors, also known as a photo-EMF sensors, enable signals to be detected coherently in the presence of dynamic optical distortions including speckle, beam-wander, and wave front distortions from atmospheric turbulence and the like. Such sensors can also sense a rapid lateral motion of an optical pattern of complex shape. Applications for photo-EMF sensors include laser remote sensing, laser communications, compensated vibrometry, laser-based ultrasound sensing and RF photonics systems.
Present photo-EMF sensors have an FOV limited to about 1.5 degrees, which stems from the small crossing angle of the interfering beams at the sensor required to produce an interference pattern with a period (spacing) on the order of the carrier diffusion length (about 60 micrometers for GaAs) for maximum response. The responsiveness of the device typically drops off rapidly for smaller crossing angles (or, equivalently, larger FOVs). The present invention can increase the FOV by an order of magnitude, without loss in responsivity (limited only by the capacitive effects). This enables the sensor to be more robust in the face of a large-angle scattered beams, spatial patterns with higher spatial bandwidth or images with finer spatial features or detail.
Also, present photo-EMF sensors have a bandwidth limited to about 100 MHz, which stems from the carrier lifetime, which is about 10 nanoseconds. The device bandwidth can be increased by reducing the carrier lifetime, for example, by ion implantation in the active regions of the detector. However, the ion implantation has a side effect in that the photo-EMF sensor exhibits a dramatic reduction in device responsiveness, defined to be the photo-current generated per watt of detected optical power per radian of differential phase shift of the interfering beams incident on the photo-EMF sensor. The present invention enables one, in essence, to recover partially the responsiveness resulting in a device with a much greater bandwidth (projected to be in the one to ten gigahertz regime), thereby making this sensor useful for myriad communication and remote sensing applications, as well as for high-frequency ultrasound and RF photonics applications.
Prior art photo-EMF sensors come in two basic device geometries. The basic sensor, using a single pair of surface electrodes and an improved sensor using an Asymmetric InterDigitated Contact (AIDC) configuration to improve the device responsiveness. The basic sensor, which utilizes a pair of relatively widely spaced electrodes (typically many mm apart), has a rather low responsiveness (10xe2x88x924 to 10xe2x88x925 A/W-radian), a limited field of view (≈1.5xc2x0) and additionally a limited bandwidth (≈100 MHz). The limited FOV of prior art devices is represented by FIG. 1. The improved structure (the AIDC configuration) improves responsiveness of the basic sensor by as much as two orders of magnitude, but does not improve its field of view, its bandwidth or its photon collection efficiency.
Briefly and in general terms, in one embodiment, the present invention provides a photo-EMF sensor which has a substrate with a semiconducting layer; a plurality of sensing regions in the layer, each sensing region including (i) a pair of electrodes disposed in, on or above the layer and (ii) an active region in the layer disposed adjacent to said pair of electrodes; and a plurality of inactive regions in the layer arranged between adjacent sensing regions. The inactive regions and the sensing regions are dosed with a desensitizing agent, such as protons, the inactive regions receiving a relatively higher dose of the desensitizing agent and the sensing regions receiving a relatively lower dose of the desensitizing agent. The notion of desensitizing the active region of a photo-EMF sensor is counter-intuitive in this art since, without employing the other aspects of this invention, it would degrade the sensor in terms of its responsiveness.
In another embodiment,. the present invention provides a method of making an interdigitated photo-EMF sensor. A substrate is made or provided having at least a layer of a semiconducting material disposed at a major surface thereof. A plurality of sensing regions are formed in said layer, each sensing region including (i) a pair of electrodes disposed adjacent said layer and (ii) an active region in said layer disposed adjacent said pair of electrodes. A plurality of inactive regions are formed in said layer arranged between adjacent sensing regions. The inactive regions and the sensing regions are dosed with a desensitizing agent, the inactive regions receiving a relatively higher dose of said desensitizing agent to thereby cause a relative higher level of defects to occur therein and said sensing regions receiving a relatively lower dose of the desensitizing agent to thereby cause a relative lower level of defects to occur.
In one embodiment the present invention may be constructed using an asymmetric Fabry-Perot structure where the active region of the photo-EMF sensor is disposed between the pair of reflecting mirrors of the asymmetric Fabry-Perot structure, with the reflecting mirror having the lower coefficient of reflection being on the light-sensing side of the photo-EMF sensor. Using an asymmetric Fabry-Perot structure improves the absorption of photons by the sensor and minimizes Fresnel reflections. Moreover, the reflecting layer having the lower coefficient of reflection, also conveniently serves as a capping layer that reduces undesirable surface recombination of carriers.